This invention relates to lubricants suitable for use in diesel engines and to a method of controlling the viscosity of such lubricants in the presence of soot.
Internal combustion engines function by the combustion of fuels which in turn generate the power needed to propel vehicles. In the case of a diesel engine, the fuel is a diesel fuel and the combustion thereof generally results in emissions from the exhausts of such vehicles which comprise three main components. These are: soot and particulate matter, carbon monoxide and nitrogen oxides (the latter will hereafter be abbreviated as NOx for convenience). To alleviate environmental concerns, research is ongoing in the petroleum industry to reduce the levels of these emissions. NOx emissions can be reduced by lowering the temperature at which the fuel is combusted in the engine. Typically this is achieved by retarding the combustion, ie by injecting the fuel shortly after the peak temperature is reached in the cylinder. However, this retarded combustion has the disadvantage that it causes more soot to accumulate in the fuel partly due to incomplete combustion of the fuel because of the lower combustion temperature, and partly due to increased soot deposition on the cylinder wall which is drawn down into to lubricant with the downward stroke of the piston. The presence of soot in the lubricant has the adverse affects of causing viscosity increase and accelerated wear. It is important that soot induced viscosity increase be controlled such that the lubricant stays within viscosity grade in order to maintain its expected performance. Several methods have been tried to alleviate this problem including the use of one or more of dispersants, metal salts and solvents which may be ethers, esters and the like. The dispersants function by forming a coating of the dispersant on the surface of soot particles and thereby minimising the tendency of the soot particles to agglomerate. However, the potency of the dispersants to perform this function, in turn, declines with time and thus, one of the methods of improving the useful life of lubricants, particularly crankcase lubricants, would be to improve the dispersancy retention capability of crankcase lubricants. This may be achieved, eg by minimising the risk of oxidation of the dispersants under the conditions prevalent in the engines during use. One such method is described in U.S. Pat. No. 5,837,657 which discloses a method of improving the performance of a sooted diesel oil and controlling soot induced viscosity increase by adding to the diesel oil a minor amount of a trinuclear molybdenum compound of the generic formula MO3SkLnQz wherein L is a ligand having organo groups, n is from 1 to 4, k various from 4 through 10, Q is a neutral electron donating compound such as eg water, amines, alcohols, phosphines and ethers, and z ranges from 0 to 5.
An object of the present invention is to achieve a method of controlling soot induced viscosity increase in lubricants by prolonging the effective performance of the dispersant additive contained in the lubricant. The dispersant is then able to disperse the soot for an extended period thereby inhibiting soot induced viscosity increase of the lubricant. In other words, an object of the present invention is to improve the dispersancy retention capability of such lubricants.
Accordingly, the present invention provides a method of controlling the soot induced viscosity increase of diesel engine lubricant compositions comprising a base oil and a dispersant by including in said lubricant composition an effective amount of an antioxidant, characterised in that the antioxidant comprises a dihydrocarbyldithiocarbamate of a metal selected from antimony, bismuth and mixtures thereof.
The lubricant compositions used in the present invention are those that comprise a major amount of a lubricating oil suitable for use in a engine crankcase, particularly a diesel engine crankcase. Thus, mineral or synthetic lubricating oils having a kinematic viscosity of 3.5 to 25 cSt at 100xc2x0 C. comprise a major portion of the lubricating compositions. Such lubricating base oils are widely available and may be any of the available base oils groups, namely Group I, II, III, IV or V. Preferably the base oil is a Group I or II base oil.
The dispersancy retention properties of such lubricant compositions is improved in accordance with this invention by including in the crankcase lubricant an added antioxidant which is a metal dihydrocarbyldithiocarbamate wherein the metal is antimony or bismuth. The antioxidant may be oil soluble or oil dispersible, but is preferably oil soluble. Such dihydrocarbyldithiocarbamates and methods of preparation thereof are claimed and described for instance in prior published U.S. Pat. No. 4,859,787 and U.S. Pat. No. 5,840,664 which are incorporated herein by reference. Thus, antimony dihydrocarbyldithiocarbamate can be prepared by the use of the following reaction:
(R)(Rxe2x80x2)Nxe2x80x94H+6CS2+Sb2O3xe2x86x922[(R)(Rxe2x80x2)Nxe2x80x94C(S)S]3Sb+3H2O
wherein R and Rxe2x80x2 are linear or branched alkyl groups.
Bismuth dihydrocarbyldithiocarbamates can be prepared by an exchange reaction between a bismuth compound such as eg a carboxylate or an alkanoate (eg bismuth neodecanoate, bismuth octanoate or bismuth naphthenate) and a metal dihydrocarbyldithiocarbamate such as eg zinc diamyldithiocarbamate. The metal dihydrocarbyldithiocarbamate used in this exchange reaction can be pre-prepared or formed in situ, for instance, by reacting a secondary amine and carbon disulphide in the presence of a metal oxide or a metal hydroxide.
The structure of the antimony or bismuth dihydrcarbyldithiocarbamates may be considered as having a ligand [xe2x80x94S2CN(R)(Rxe2x80x2)] wherein the dihyrocarbyl groups, R and Rxe2x80x2 impart oil solubility to the antimony and bismuth compounds. In this instance, the term xe2x80x9chydrocarbylxe2x80x9d denotes a substituent having carbon atoms directly attached to the remainder of the ligand and are predominantly hydrocarbyl in character within the context of this invention. Such substituents include the following:
(1) hydrocarbon substituents, ie, aliphatic (for example alkyl or alkenyl), alicyclic (for example cycloalkyl or cycloalkenyl), aromatic-, aliphatic- and alicyclic-substituted aromatic nuclei and the like, as well as cyclic substituents wherein the ring is completed through another portion of the ligand (that is, any two indicated substituents may together form an alicyclic group);
(2) substituted hydrocarbon substituents, ie, those containing nonhydrocarbon groups which, in the context of this invention, do not alter the predominantly hydrocarbyl character of the substituent. Those skilled in the art will be aware of suitable groups (eg halo (especially chloro), amino, alkoxyl, mercapto, alkylmercapto, nitro, nitroso, sulphoxy etc.); and
(3) hetero substituents, ie, substituents which, while predominantly hydrocarbon in character within the context of this invention, contain atoms other than carbon present in a chain or ring otherwise composed of carbon atoms.
The hydrcarbyl groups are preferably alkyl (e.g, in which the carbon atom attached to the remainder of the ligand is primary, secondary or tertiary), aryl, substituted aryl and ether groups.
Importantly, the hydrocarbyl groups of the ligands should be such that they have a sufficient number of carbon atoms to render the corresponding antimony or bismuth dialkyldithiocarbamate soluble or dispersible in the oil to which it is added. The total number of carbon atoms present among all of the hydrocarbyl groups of the compounds"" ligands is suitably at least 21, preferably at least 25 and preferably at least 30, typically e.g., 21 to 800. For instance, the number of carbon atoms in each hydrocarbyl group will generally range from 1 to 100, preferably from 1 to 40 and more preferably from 3 to 20.
The antioxidant in the compositions of the present invention suitably also include at least one of a phenolic antioxidant and an aminic antioxidant. Among the phenolic antioxidants, hindered phenols are preferred.
Thus, according to a further embodiment, the present invention is a diesel engine lubricant composition comprising a base stock, a dispersant and an antioxidant comprising two or more of (a) an antimony dihydrocarbyldithiocarbamate, (b) a bismuth dihydrocarbyldithiocarbamate, (c) a phenolic compound and (d) an aminic compound. The nature of the antimony and bismuth dihydrocarbyldithiocarbamates in these compositions is described above. The nature and amounts of the phenolic and aminic compounds in said compositions are described below.
Examples of such phenolic compounds include inter alia:
4,4xe2x80x2-methylene bis(2,6-di-tert-butylphenol)
4,4xe2x80x2-bis(2,6-di-tert-butylphenol)
4,4xe2x80x2-bis(2-methyl-6-tert-butylphenol)
2,2xe2x80x2-methylene bis(4-ethyl-6-tert-butylphenol)
2,2xe2x80x2-methylene bis(4-methyl-6-tert-butylphenol)
4,4xe2x80x2-butylidene bis(3-methyl-6-tert-butylphenol)
4,4xe2x80x2-isopropylidene bis(2,6-di-tert-butylphenol)
2,2xe2x80x2-methylene bis(4-methyl-6-nonylphenol)
2,2xe2x80x2-isobutylidene bis(4,6-dimethyl phenol)
2,2xe2x80x2-methylene bis(4-methyl-6-cyclohexylphenol)
2,6-di-tert-butyl-4-methylphenol
2,6-di-tert-butyl-4-ethylphenol and
2,4-dimethyl-6-tert-butylphenol
The phenolic antioxidants are preferably compounds which have the following structures: 
wherein, in the formulae (I)-(IV) above, R1, R2, and R3 are the same or different alkyl groups having 3-9 carbon atoms and x and y are integers from 1 to 4.
Suitable amine antioxidants for use in the compositions of the present invention are diaryl amines, aryl naphthyl amines and alkyl derivtives of diaryl amines and the aryl naphthyl amines. Specific examples of the aminic compounds that may be used in the compositions of the present invention include inter alia:
Monoalkyldiphenyl amines such as eg monooctyldiphenyl amine and monononyl diphenyl amine; dialkyldiphenyl amines such as eg 4,4xe2x80x2-dibutyldiphenyl amine, 4,4xe2x80x2-dipentyldiphenyl amine, 4,4xe2x80x2-dihexyldiphenyl amine, 4,4xe2x80x2-diheptyldiphenyl amine, 4,4xe2x80x2-dioctyldiphenyl amine and 4,4xe2x80x2-dinonyldiphenyl amine; polyalkyldiphenyl amines such as eg tetra-butyldiphenyl amine, tetra-hexyldiphenyl amine, tetra-octyldiphenyl amine and tetra-nonyldiphenyl amine; the naphthylamines such as eg xcex1-naphthylamine and phenyl-xcex1-naphthylamine; butylpheny-xcex1-naphthylamine, pentylphenyl-xcex1-naphthylamine, hexylphenyl-xcex1-naphthylamine, heptylphenyl-xcex1-naphthylamine, octylphenyl-xcex1-naphthylamine and nonylphenyl-xcex1-naphthylamine. Of these, dialkyldiphenyl amine and naphthylamines are preferable.
In general the antioxidant which comprises the dihydrocarbyldithiocarbamates of antimony and/or bismuth and optionally phenolic and/or aminic compounds will form a minor component of the total lubricant composition. For example, the antimony or bismuth dihydrocarbyldithiocarbamate typically will comprise about 0.05 to about 3 wt %, preferably about 0.1-2% of the total composition, ie the antimony and/or bismuth metal is suitably present in an amount of about 50-2000 ppm, preferably from about 200-1500 ppm of the total composition. The optional phenolic and/or aminic compounds are suitably present in an amount of about 0.1 to about 3 wt % of the total composition.
It has also been found that if the weight ratio of antimony and/or bismuth dihydrocarbyldithiocarbamate to the phenolic or aminic compound in the antioxidant is in the range of about 80:20 to about 20:80, optimum dispersancy retention can be achieved by these combined additives of the present invention.
It is particularly preferred that the antioxidant comprises in addition to the antimony and/or bismuth dihydrocarbyldithiocarbamate a mixture of hindered phenols and a diaryl amine in a weight ratio ranging from about 20-80:10-60:10-60 respectively.
Optionally, the antioxidants may be combined with a carrier liquid in the form of a concentrate. The concentration of the combined antioxidants in the concentrate may vary from 1 to 80% by weight, and will preferably be in the range of 5 to 10% by weight.
Any of the conventional dispersants used hitherto in the lubricating compositions may also be used in the compositions of the present invention. Examples of these include the polyalkylene succinimides, Mannich condensation products of polylalkylphenolformaldehyde polyamine and borated derivatives thereof. However, it is preferable to use ashless dispersants such as the ashless succinimides, especially the polyisobutenyl succinimides of a polyamine such as eg tetraethylenepentamine, benzylamine ashless dispersants, and ester ashless dispersants. The dispersants are generally used in the compositions of the present invention in an amount ranging from about 1-10% by weight based on the total weight of the lubricant composition, preferably from about 4-8% by weight.
In general, these lubricating compositions may include additives commonly used in lubricating oils especially crankcase lubricants, such as antiwear agents, detergents, rust inhibitors, viscosity index improvers, extreme-pressure agents, friction modifiers, corrosion inhibitors, emulsifying aids, pour point depressants, anti-foams and the like.
A feature of the lubricant compositions of the present invention is that the presence therein of antimony and/or bismuth dihydrocarbyldithiocarbamate as an antioxidant provides unexpected improvement in oxidation control, viscosity increase control and dispersancy retention over compositions which contain conventional organomolydenum compounds such as the corresponding dinuclear molybdenum dihydrocarbyldithiocarbamates.